MS Word Documents Spreading .Net RAT Malware

A malicious MS Word document, titled “eml_-_PO20180921.doc,” has been found in the wild, and according to researchers at Fortinet's FortiGuard Labs, the document contains auto-executable malicious VBA code.

Victims who receive and open the document are prompted with a security warning that macros have been disable. If the user then clicks on “enable content,” the NanoCore remote access Trojan (RAT) software is installed on the victim’s Windows system.

According to FortiGuard Labs, the NanoCore RAT was developed in the .Net framework back in 2013. Despite its continued use, the author was convicted by the FBI and sentenced to nearly three years in prison. Researchers captured a sample of this latest version (1.2.2.0), which uses NanoCore to execute malicious behavior.

Spreading through phishing campaigns that dupe victims into opening the document, the malware is downloaded from www.wwpdubai.com. Once executed, the VBA code downloads and saves an EXE file from the URL.

“I loaded CUVJN.exe with the .Net debugger dnSpy. Tracing from its main function, we can see that it loads numerous data blocks from its resource section, and then puts them together and decrypts them,” wrote researcher Xiaopeng Zhang.

In order to trace the main functions, researchers loaded CUVJN.exe with the .Net debugger dnSpy and found that it loads, puts together and then decrypts multiple data blocks from its resource section in order to get to a new PE file.

“According to my analysis, the decrypted .Net program is a daemon process. Let’s continue to trace it from its main() function. At first, it creates a Mutex and checks if the process already exists to ensure only one process of this program is running. Next, it checks if Avast is running on the victim’s system by detecting whether the “snxhk.dll” module is loaded or not. If so, it keeps waiting until it has been unloaded. Avast is an AntiVirus software, and “snxhk.dll” is one of its modules,” Zhang wrote.

Unfortunately, .dll is a daemon process, which Zhang said he was not able to kill because it has a “ProtectMe” class, though he does provide steps for removing the malware.